Back pack



FIG. 1 is a perspective view of a back pack showing my new design;

FIG. 2 is a rear elevational view;

FIG. 3 is a top plan view;

FIG. 4 is a side elevational view; and

FIG. 5 is a bottom plan view.

The partial showing of surface shading indicating a fabric texture is understood to be continuous over those surfaces where shown. 

The ornamental design for a back pack, as shown and described. 